When a user equipment (UE), such as a cellular phone, is used to dial a 911 emergency call center, government regulations call for an ability to determine the location of the UE so that the location may automatically be sent to the 911 emergency call center. Many UEs are equipped with a global positioning system (GPS) receiver that can accurately determine the UE location. However, if the link to GPS satellites is weak or the UE does not have GPS capability, then another method of position determination is called for.
One such method is a technique called observed time difference of arrival (OTDOA). This technique operates by transmitting a special physical channel called a position reference signal (PRS) to the UE from 3 or more base stations such as evolved node Bs (eNBs) in a long term evolution (LTE) communication network. The PRS includes precise timing information. The UE observes the relative arrival times of the signals from nearby eNBs and reports the reference signal time difference (RSTD) observations to a particular serving eNB to which the UE is actively attached. The serving eNB conveys the report of observations to a location server which computes the position estimate of the UE using triangulation. The OTDOA mechanism relies upon accurate knowledge of the antenna locations of the eNBs, an eNB time reference relative to the other eNBs, and the observed time of arrival of a PRS signal at the UE. An error in any of these parameters degrades the position estimate of the UE.
The time of arrival (TOA) of the PRS is presumed to be received from a line of sight (LOS) direction, i.e., a direct path from an antenna of an eNB. Depending on the propagation environment, the LOS path may be attenuated due to an obstruction, such that a relatively stronger signal from the antenna may arrive from the eNB at the UE via a path that includes a reflection. This signal may drown out the LOS signal and may convey inaccurate TOA information.
The accuracy of the estimate of the location of the UE by the location server may be adversely affected by non-LOS propagation of the PRS received at the UE such as may result from reflected multipath signals.